There are a variety of flat panel display vendors and thus different types of displays for laptop and notebook computer systems. Some displays require a positive operating voltage while others require a negative operating voltage. As far as is known, no vendors require both positive and negative voltages. Many manufacturers design laptop and/or notebook computers with dual output power supplies providing both a positive and a negative voltage so that either a positive or a negative flat panel display may be used. Several problems exist with such dual output power supplies. A dual output capability increases the cost of the computer, and also increases the difficulty of maintaining tight and predictable load regulation of the output voltage polarity being used.
Some manufacturers of laptop and notebook computers provide two separate power converters each having a separate feedback path. Obviously, two separate power converters is relatively expensive considering that only one is used. Another solution is a single power converter with an inherently tight output low regulation figure of merit. A forward converter is typically used in the single power converter configuration using a coupled inductor output stage, since forward converters are inherently easier to regulate. However, forward converters are relatively sophisticated and thus expensive to design and manufacture. A less expensive flyback converter could be used with a complex dual input feedback path which sums both the polarity outputs. This particular flyback solution, however, is less than optimal since the desired load regulation figure of merit is very difficult to achieve.
It is desired, therefore, to provide a relatively inexpensive power supply having both a positive and negative operating voltage to supply power to either a positive or negative flat panel display. The power supply should maintain the optimal load regulation figure of merit regardless of which output is loaded.